This invention relates to graphics displays, in particular though not exclusively digital storage oscilloscopes.
Digital storage oscilloscopes have two display modes, usually referred to as REFRESH and ROLL display modes. The REFRESH mode display is used in conjunction with trigger operations in which the display is triggered by a predetermined event from an internal or external source. Problems have arisen in the REFRESH mode of operation with low time base rates in that the periods required for display of operation following a trigger event are excessive.
In order to explain this problem, reference will first be made to FIGS. 1 and 2 which are block diagrams of parts of a digital storage oscilloscope relevant to the ROLL mode and REFRESH mode, respectively. Referring firstly to FIG. 1 which shows the parts of a system relevant to the ROLL mode, an acquisition system 2 is provided for gathering information from a source to be examined and providing this information in digital form to an acquisition buffer 4 where it is temporarily stored before storing in a display store 6 at an appropriate position. Display store 6 is controlled by an addressing system comprising a write counter 8, a display counter 10, an adder 12 and an address multiplexer store 14. The display store 6 supplies information to a buffer 16 from whence the information is fed to the display system 18. Thus in operation, a multiplexing operation occurs so that data is written into the display store 6 from the acquisition buffer 4 under the control of write counter 8 and at the same time information is fed to the display buffer 16 for display by the display system 18 under the control of display counter 10. Display counter 10 in addition determines the position on the X coodinate of the cathode ray screen of the current bit of information to be displayed. Thus the display counter 10 provides the X address for the display system, sweeping the display from left to right with increasing count. By adding the current value of the write counter 8 to the display counter 10 to generate the address of the data for display, the ROLL mode is achieved. In the ROLL mode the newest data received from the acquisition buffer 4 is displayed on the righthand side of the screen whereas the oldest information in the display buffer 10 is lost on the lefthand side of the screen. Thus the buffer will roll from right to left as new data is entered from the acquisition system 2.
Referring now to FIG. 2 the parts of the system relevant to the REFRESH mode are shown. Similar or corresponding parts to those shown in FIG. 1 are indicated by the same reference numeral. It will be noted two buffers are provided, a post-trigger acquisition buffer 20 and a pre-trigger acquisition buffer 22 with a transfer buffer 24 connected therebetween. These buffers supply a pre-trigger store 26 which is controlled by a pre-trigger counter 28 which addresses the store 26. In operation, when the digital storage oscilloscope is switched to a trigger mode of operation it is necessary to collect two sets of information, namely information relating to a period or cycle immediately before the trigger event in pre-trigger buffer 22 and then information relating to a period or cycle immediately after the trigger in a post-trigger buffer 20. In a REFRESH mode the display remains static until the first trigger following a memory flushing period. During this so-called memory flushing period information is gathered from the period or cycle preceding the trigger and the trigger mode of operation is disabled until there is sufficient data within the pre-trigger store 26 to provide a display on display system 18. Thus in operation information is firstly stored in buffer 22 and then transferred to pre-trigger store 26 under the control of pre-trigger counter 28. When the pre-trigger data has been acquired and a trigger event occurs, information is transferred via transfer buffer 24 from pre-trigger store 26 to display store 6 under the control of a transfer counter 30. This information is displayed in system 18 under the control of display counter 10 and at the same time information from acquisition system 2 is entered into post-trigger buffer 20 and thence into display store 6 under the control of write counter 8. Thus a three-way multiplexing system is set up to permit display of the pre-trigger information whilst gathering post-trigger information. Thus once the post-trigger information has been acquired, it is then subsequently displayed in display system 18.
The problem with this arrangement is that in the REFRESH mode the time taken to fill pre-trigger buffer 22 with a cycle or period of information can be excessive at very low time base rates. For example 50% pre-trigger at 10 seconds per centimetre will require a 50 second period in which buffer store 22 is filled and a trigger event occurs before the display can be updated. During this period the operator cannot observe any activity and this is clearly a disadvantage in terms of being an inconvenience to the operator.